All commercial polyethylene resins used in high-stalk extrusion equipment have a relatively broad MWD as indicated by MFR values of 80-200. Although resins with relatively broad MWD exhibit good processability on high-stalk extrusion equipment, their film toughness properties, such as tear strength, are relatively poor.
In contrast, polyethylene resins with a relatively narrow MWD are not suited for high-stalk film extrusion equipment. We found, however, that even polyethylene resins with a relatively narrow MWD can be processed with such equipment if the resins contain a significant fraction of polymer molecules with very high molecular weights. Moreover, such resins exhibit excellent film properties such as impact strength and tear resistance.
One of the measures of MWD of a LLDPE or MDPE resin is its melt flow ratio (MFR), which is the ratio of the high-load melt index (HLMI or I.sub.21) to the melt index (MI or I.sub.2) for a given resin: MFR=I.sub.21 /I.sub.2. The MFR value is an approximate indication of MWD of a polymer: the higher the MFR value, the broader the MWD. Common polyethylene resins for film applications usually have relatively low MFR values, e.g., of about 15 to about 30.
It is an object of the present invention to provide a high-activity catalyst for copolymerization of ethylene and alpha-olefins yielding products with a multimodal, relatively narrow MWD as indicated by MFR values in the 28-70 range. It is an additional object of the present invention to provide a catalytic process for copolymerizing ethylene with alpha-olefins which yields products with a bimodal MWD at high productivity.